Effect of Acid−Base Equilibria on the Donnan Potential of Layer-by-Layer Redox Polyelectrolyte Multilayers

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Abstract
In polymer films carrying an excess of fixed charge the electrostatic penalty to bring ions of same charge from the bathing electrolyte into the film sets a membrane potential (Donnan Potential) across the film−electrolyte interface. This potential is responsible for the ionic permselectivity observed in polyelectrolyte membranes. We have used electrochemical measurements to probe the dependence of the Donnan potential on the acid−base equilibrium in layer-by-layer self-assembled polyelectrolyte multilayers. The voltammperogram peak position of the Os(III)/Os(II) couple in self-assembled polyelectrolyte multilayers comprised of poly(allylamine) derivatized with Os(bpy)2PyCl+ and poly(vinylsulfonate) was recorded in solutions of increasing ionic strength for different assembly and testing solution pH. Protonation−deprotonation of the weak redox poly(allylamine) changes the fixed charge population in the as prepared (intrinsic) self-assembled redox polyelectrolyte multilayers. For films assembled in solutions of pH higher than the test solution pH, the Donnan plots (Eapp vs log C) exhibit a negative slope (anionic exchanger) while for films assembled at lower pH than that of the test solution positive slopes (cationic exchanger) are apparent. The ion exchange mechanism has been supported by complementary electrochemical quartz crystal microbalance. X-ray photoelectron spectroscopy and infrared reflection−absorption spectroscopy experiments demonstrated that the as prepared films have a memory effect on their protonation state during assembly, which leads to the observed dependence of the Donnan potential on the adsorption pH.